1. Field of the Invention
This invention relates in general to steam generators for nuclear power plants and more particularly, to vertical, U-tube steam generators having both loose parts and sludge collectors.
2. Description of Related Art
A nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, a dividing plate that cooperates with the tube sheet and a channel head forming a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle is in fluid communication with the primary fluid outlet header. The steam generator secondary side comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber made up of the shell on the outside and a wrapper on the inside, and a feedwater ring disposed above the U-like curvature end of the tube bundle.
The primary fluid having been heated by circulation through the reactor enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle the primary fluid is conducted through the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header, through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced to the steam generator secondary side through a feedwater nozzle which is connected to a feedwater ring inside the steam generator. Upon entering the steam generator, the feedwater mixes with water returning from steam separators, called the recirculation stream. This mixture, called the downcomer flow is conducted down the annular chamber adjacent to the shell until the tube sheet near the bottom of the annular chamber causes the water to reverse direction passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the water is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to the water surrounding the tubes causing a portion of the water surrounding the tubes to be converted to steam. The steam then rises and is conducted through a number of moisture separators that separate entrained water from the steam, and the steam vapor then exits the steam generator and is typically circulated through electrical generating equipment to generate electricity in a manner well known in the art.
Since the primary fluid contains radioactive materials and is isolated from the feedwater only by the U-tube walls, the U-tube walls form part of the primary boundary for isolating these radioactive materials. It is, therefore, important that the U-tubes be maintained defect-free so that no breaks will occur in the U-tubes that will cause radioactive materials from the primary fluid to enter the secondary side; an undesirable result.
Loose parts enter the steam generator through the feedwater stream and can cause damage to the heat transfer tubes. Sludge can also enter the steam generator through the feedwater stream and can also cause damage to the heat transfer tubes. The size of sludge particles are in the range of micrometers while loose parts are in the range of inches. Sludge tends to deposit on tube surfaces and eventually lead to chemical concentrations that cause tubing corrosion. The damage caused by loose parts and sludge can result in having to plug or repair the damaged tubes to avoid contamination of the secondary fluid. In extreme cases, the damage can lead to a tube leak and forced outage with significant expense to the plant. Therefore, it is important to prevent foreign objects and minimize sludge from entering the steam generator and/or to remove the loose parts and sludge from the steam generator before tube damage occurs.
Prior attempts to prevent steam generator loose parts from reaching the tube bundle have focused on a sieving action. For example, spray nozzles with small holes have been attached to the feedwater distribution ring to trap loose parts. Although such spray nozzles have succeeded in trapping larger parts, small parts may pass through the holes in the nozzles due to their size. Such loose parts, e.g., pieces of metal rope or rods, have caused tube damage in operating steam generators.
Co-pending application Ser. No. 11/563,742 filed on Nov. 28, 2006 (NSD 2005-013) proposes an improved loose parts collector that employs a weir having a vertical wall that surrounds at least a portion of a lower deck plate of the steam generator. The weir has a radially inwardly extending lip affixed to the upper end of the wall that traps loose parts as the combined feedwater and recirculation stream flows over the weir and down the downcomer between the wrapper and the shell. This proposed arrangement provides a substantial improvement to the collection of loose parts.
Previously, sludge collectors had been used which generally provided collector boxes that sat on top of the lower deck plate. The collector boxes typically had centrally disposed water inlet holes and peripherally disposed water outlet holes around the circumference of the collector box lid. The sludge collector box would draw a portion of the feedwater from the feedwater ring and the recirculation stream into the collector box through the water inlet holes on the lid. The water passes through the collector towards the periphery of the box in extremely slow motion providing the particles time to settle to the floor of the collector box while the water continues exiting the collector box through the outlet holes at the edge of the lid of the collector box. The water enters the collector due to a pressure differential between the inlet and outlet holes on the lid of the collector box. The pressure differential is due to the fact that the fluid flow near the inlet holes is relatively quiescent compared to the flow over the outlet holes where the remaining portion of the feedwater from the feedwater ring and the recirculation stream is rushing into the downcomer annulus at a relatively high velocity. Therefore, fluid static pressure is relatively high at the inlet holes and low at the outlet holes and a pressure differential is developed between them that is the driving force to draw the water into the box of the sludge collector.
The loose parts collector proposed by application Ser. No. 11/563,742, filed Nov. 28, 2006 (NSD 2005-013) consisting of the weir having a circular vertical skirt around the edge of the lower deck together with a horizontally inwardly extending lip complicates and to a degree compromises the operation of the sludge collector. The water pool on top of the lower deck plate receives both recirculated water and feedwater. The recirculated water comes from the separated water from the primary separator. The separated water is at the saturation temperature. The feedwater discharges into the water pool via spread tubes or J-tubes of the feedwater ring. The feedwater is subcooled, about 100° F. below the saturation temperature. The separated, saturated water will mix with the subcooled feedwater in the water pool. The mixed water is still subcooled (about 20° F. below the temperature of the saturated water) and flows over the edge of the lower deck plate and downward along the downcomer annulus.
The water mass in the water pool over the lower deck plate is large compared to the mass of the feedwater flow and the recirculated, saturated water flow. Thus, the fluid flow in the water pool and on the lower deck plate is generally slow except for the limited local zones where water enters and leaves the pool. Therefore, loose parts will not travel far prior to falling on lower deck plate. The weir will therefore retain the fallen loose parts. However, the weir also restricts flow at the outlet of the sludge collector and, thus, pressure is essentially uniform on top of the lower deck. In other words, both the center and edge are at the same high pressure. Thus, because of the weir of the loose part collector, the pressure differential between the inlet and the outlet holes of the sludge collector disappears and thus there would be no flow within the sludge collector box when combined with such a loose parts collector. In other words, the installation of the loose parts collector described in co-pending application Ser. No. 11/563,742 (NSD 2005-013) has the potential to destroy the function of the sludge collector.
Accordingly, it is an object of this invention to provide an improved loose parts collector and sludge collector that will not impede the performance of either.
Furthermore, it is an object of this invention to provide such a dual sludge and loose parts collector that will efficiently collect both loose parts and sludge.
Additionally, it is an object of this invention to provide such a loose parts and sludge collector that will not impede the efficiency of the steam generator.